1. Field of the Invention
The present invention relates to a method for preparing a composite membrane for separation of hydrogen gas and more particularly, to a method for preparing the composite membrane with better permeation and separation coefficient on hydrogen gas, prepared in such a manner that an alloy consisting of a palladium complex compound and transition metal is electroplated on a modified porous support under vacuum, thereby forming the thin plated-layer without pin-holes.
2. Description of the Related Art
Generally, it is advantageous to use a material with larger permeation and separation coefficients as a membrane for separation of a hydrogen gas. When the hydrogen gas is separated by a membrane derived from an organic polymer, the membrane manifests some poor physical properties such as its thermal stability, chemical resistance and durability. In particular, the selectivity of a membrane to a certain gas tends to be lowered as its gas permeability becomes larger, and thus a highly purified hydrogen cannot be obtained.
Meanwhile, a palladium-based separation membrane has a higher solubility and diffusion property to the hydrogen gas so that the selective separation of hydrogen gas may be available through its dense surface. In this context, some metals such as palladium have been mainly used so as to obtain a highly purified hydrogen from a mixing gas containing hydrogen.
However, the use of pure palladium membrane for the hydrogen separation is restricted, as hydrogen embrittlement caused by the transition between the xcex1- and xcex2-phase occurs in membranes in contact with hydrogen at temperature below 300xc2x0 C. and pressure below 2 HPA. Since the lattice constant of the xcex2-phase is at least 3% larger than that of the xcex1-phase, the nucleation and growth of the xcex2-phase cause strains in the metal and thereby the embrittlement of the material.
To comply with this matter, a method of using an alloy containing palladium and other metals has been suggested instead of use of pure palladium; With its proper prevention of any hydrogen saturation and interaction between a metal plated in the form of substitution and hydrogen atom, a palladium-alloy membrane may serve to prevent the regular arrangement of hydrogen atoms within the lattice and thus, any formation of a beta phase does not occur. The palladium-alloy membrane may further enhance the selectivity to hydrogen gas and durability, compared with a separation membrane derived from pure palladium. In particular, in order to improve the performance of membrane, a method of coating palladium thinly on a porous support has been under consideration and some studies on such composite-type separation membrane has been mainly conducted.
As disclosed in U.S. Pat. No. 2,773,561, the use of pure palladium or palladium-based alloy membrane with some metal in the absence of the composite-type separation membrane has some disadvantages in that a) this method is uneconomical due to an excess use of palladium in a membrane, and b) since the thickness of membrane should be in the range of 25-150 xcexcm to endure the temperature and pressure, its larger thickness may lower the permeation rate.
In this respect, it is recommended that a palladium composite membrane containing a thin palladium alloy membrane on its support, be employed as a separation membrane for hydrogen gas instead of palladium metal membrane or thin palladium alloy membrane.
Examples of the existing materials used for a support of a palladium composite membrane includes glass (Chem. Lett., 10, 1687(1988)), inorganic materials such as porous ceramics (Ind. Eng. Chem. Res., 33, 616(1994)); J. Memb. Sci., 56,315(1991)) and stainless steel (J. Memb. Sci., 181(1993)). Whereas the weak strength of the glass and porous ceramics are susceptible to easy destruction and the sealing between a separation membrane and permeation cell may not be made available, a porous support derived from a stainless steel has several advantages such as a) lower material cost, b) little occurrence of corrosion and crack, c) easier processing, and d) higher mechanical strength for modulation.
In addition, examples of the plating methods designed to prepare a palladium composite membrane by means of coating palladium metal membrane and palladium alloy membrane on a porous support includes a) electroless plating (J. Memb. Sci., 77, 181(1993); Ind. Eng. Chem. Res., 32,3006(1993)), b) chemical vapor deposition (Ind. Eng. chem. Res. 33, 616(1994); J. Memb. Sci., 120,261(1996)) and c) sputtering (J. Memb. Sci., 94,299(1994): J. Memb. Sci. 104,251(1995)).
The electroless plating method comprising a variety of multi-pretreatment steps and several repeated activation steps at the surface of support has several disadvantages in that a) more prolonged treatment time is required, b) the thickness of film may be controllable due to continuous plate growth, and c) the excellent capacity of membrane may be hardly expected, since a membrane with a thickness of 10-20 xcexcm should be prepared to form a pinhole-free layer.
The chemical deposition (CVD) method has also been used to coat thin films of thickness below 5 xcexcm. Although the CVD method could prepare highly selective films, it required too high purity of constituents and strict conditions in view of an economical aspect. Also, since the CVD method cannot form an alloy, the method forms xcex2-phase which induces hydrogen embrittlement.
The sputtering method has little separation effect of hydrogen gas via a palladium layer due to difficulty in forming defect-free film.
Therefore, an object of this invention is to provide a method for preparing a composite membrane for separation of hydrogen gas with improved permeability and separation capability to hydrogen gas, wherein a palladium alloy compound is electroplated on a porous support; instead of using palladium, a palladium complex compound is applied so as to lessen the difference of standard electropotential between palladium and transition metal used as alloy metal; the surface of porous support is modified to form a homogeneous, thin and pinhole-free layer; in particular, during electrodeposition, the thickness of an electrodeposited layer, so formed in the presence of vacuum, is thinly adjusted to further enhance the permeation and selectivity of hydrogen gas from a mixing gas containing hydrogen.
This invention is characterized by a method for preparing composite membrane for separation of hydrogen gas using palladium, wherein an alloy containing palladium and transition metal is electroplated on a porous support.
This invention is explained in more detail as set forth hereunder.
This invention relates to a method for preparing a composite membrane where a palladium alloy complex is the form of homogenous and thin membrane is located, without any pin-hole formation, at one surface of porous support having possibly homogeneous micropores, to the extent that the diffusion of gas may not be affected. To this end, some major characteristics of this invention are that;
a palladium complex compound is used;
the surface of support is modified to thinly adjust an alloy layer without any pin-hole formation;
an electroplating method under vacuum state is applied.
According to this invention, an electroplating method is applied so as to form a palladium alloy membrane on the porous support. Under the conventional method, this method is not only simply and easy to adjust a thickness of the composite membrane, but also economical in that less time is required than the electroless plating and chemical vapor deposition.
In case of applying a general electroplating method designed to form a palladium alloy membrane, the amount of palladium from the composition of alloy is not easily adjusted due to big difference of standard electropotential between palladium and transition metal. For example, the standard electropotential of palladium and nickel used as a transition metal is 0.987V and xe2x88x920.250V, respectively, and a difference of standard electropotential between both is 1.237V. In this regard, when a direct current is given, the plating amount of palladium is larger than a transition metal due to more rapid plating rate of the former and thus, an alloy with a desired chemical composition cannot be prepared.
To overcome such shortcomings associated with the application of electroplating method, this invention has introduced a method of electroplating an alloy derived from a palladium complex compound and transition metal on a porous support; more specifically, the whole molecules present in the support are electrically neutralized to lower a difference of standard electropotential, thus being easy to adjust a composition of alloy, using the palladium complex compound.
According to this invention, the method for preparing a composite membrane for separation of hydrogen gas is explained in more detail as follows:
The first step is to prepare a palladium complex compound. In case of the complex compound using palladium and ligand, it is preferred to select the ligand which is strongly coordinate with palladium in aqueous solution and which does not form any other complex compound. The typical examples of ligand include ethylene diamine, diethylene diamine, tetraammonia and diammonia; among them, it is more preferred to select ethylene diamine. The manufacturing process of the palladium complex compound is based on the existing method (Hydem Gijutsu, M. Matsunaga, M. Hara, A. Ablimit, Y. Tsura and K. Hosokawa, 43(10), 987(1992)).
The second step is to prepare an alloy using the palladium complex compound, so formed above and transition metals.
The examples of the transition metal include VIII group elements such as Pt, Rh, Ir, Fe, Co and Ni; IB group elements such as Cu, Ag and Au; VIA group elements such as Cr, Mo and W; IVA group elements such as Ti and Zr; and VA group element such as Ta, Nb and V. It is preferred to select VIII group elements, more preferably Ni.
Also, it is preferred that to rapidly facilitate the alloy between the palladium complex compound and transition metal, the transition metal is in the form of ion compound.
Further, it is preferred that the amount of palladium is in the ratio of more than 50% by weight to the total weight of the alloy and more preferably, the ratio of the palladium to the transition metal is in the range of 50:50-90:10 wt %.
The process for manufacturing the composite membrane designed to electroplate palladium alloy on a porous support is as follows:
In order to manufacture a homogeneous membrane without pin-holes based on the conventionally available electroplating method, the thickness of an electroplated layer should be more than 5 xcexcm but such range of thickness may induce a poor permeability of membrane. Under such circumstances, an electroplated layer with more thinly adjusted thickness may improve the permeability coefficient of hydrogen gas.
According to this invention, the vaccumization at the opposite side of the electroplated layer during electroplating may deeply prevent the pores present at the surface of support, while reducing the thickness of the electroplated layer. The thickness of the electroplated layer may vary differently depending on the intensity of vaccumization. In this context, it is preferred that the vacuumization is carried out in the range of 500-650 torr.
The composite membrane manufactured by the above electroplating process of this invention exhibits significantly excellent permeation property to hydrogen gas as well as better permeability and selectivity, compared with the conventional membrane using the common electroplating method.
Examples of some porous support materials according to this invention may include conductive metals such as stainless steel including ceramic or glass coated with a conductive material; hence, it is preferred to use a porous support derived from conductive metals and any of porous support having a flat or tube membrane may be employed. However, when a palladium alloy is directly electroplated on the surface of such porous support, the membrane structure may be inadequate to separation of hydrogen gas due to the relative thickness of the electroplated layer and presence of pin-holes. Thus, the surface of the support should be modified, while readjusting the pore size.
Generally, the average pore size of the porous support used for separation of gas is in the range of 0.01-1 xcexcm; in this case, the separation of gas is controlled by the Knudsen diffusion which is in inverse to the square root of molecular weight of permeating gas molecules and hence, the selectivity of hydrogen to nitrogen gas is about 3.7.
According to this invention, some conductive transition metals such as nickel are used to modify the surface of a porous support. In addition, some other conductive materials with a same effect may be used; for example, fine powders of conductive carbon are coated in the surface of the support. When the surface is coated with fine nickel powders, the separation coefficient of hydrogen and nitrogen is measured about 2.0-2.7. This reflects that the surface is somewhat modified but relatively large pores still exist on the surface of support. Meanwhile, when the surface coated with fine granules of nickel is copper plated, the separation coefficient of hydrogen and nitrogen is measured at about 3.5-3.7, and this reflects that the adhesive powder between a support and electroplated layer is improved, while the pore size of support becomes smaller. Thus, a palladium alloy is electroplated on a porous support to obtain an electroplated layer in the form of a thin membrane without pin-holes.
One embodiment of this invention designed to electroplate a palladium alloy on a stainless steel support by an electroplating method using a vacuum, is explained in more detail.
The surface of the stainless steel support, having a pore size of 0.2-0.5 xcexcm, is coated with fine nickel powders less than 1 xcexcm in size and heated at 700-850xc2x0 C. for 4-6 hours; hence, the coating amount of nickel powders is in the range of 0.003-0.01 g per unit area.
Then, the surface of the support is washed with distilled water to remove some dust and impurities and treated with a sulfuric acid solution for activation of the surface. The surface is copper-plated in the range of 200-1,000 m{circumflex over ( +L )} for about 3-30 minutes using a bath containing diluted copper cyanide.
The support, so modified from the above procedure, is fixed to a plating plate cell, immersed in a palladium alloy bath and electroplated, while reducing the pressure at the opposite side of electroplate by less than 650 torr.
In order to assess the performance of the composite membrane for separation of hydrogen gas, so manufactured from the above-mentioned manufacturing method, the permeability was measured by a gas-permeability apparatus at normal state; the adhesion between the composite membrane and permeation cell was performed by a graphite ring; temperature was adjusted by an electric furnace at an outer cell. With such equipment, the composite membrane, which is thinly coated with a palladium alloy compound and transition metal on its support, may be used as a selective membrane for separation of hydrogen gas; hence, the higher temperature is, the larger permeation rate of hydrogen gas becomes and the higher pressure difference of hydrogen gas between the membrane, the larger permeation rate of hydrogen gas also becomes.
The following specific examples are intended to be illustrative of the invention and should not be construed as limiting the scope of the invention as defined by the appended claims.
Preparation of a Plating Solution Derived from Palladium Alloy
Ethylene diamine was added to 5 g of palladium; bichloride dissolved in 200 ml of distilled water in the molar ratio of 2.2, stirred the mixture and filtered off the non reactants, while passing the vapor. Then, 35% hydrochloric acid was added to the filtered solution in the molar ratio of 2.2, stirred the mixture for about 30 minutes and cooled at 4xc2x0 C. for 24 hours for storage. Thereafter, a palladium complex compound, so precipitated, was filtered off and dried by a vacuum dryer. Hence, the remaining ethylene diamine was separated via a reaction of ethylene diamine with hydrochloric acid to form an ammonium salt, while the palladium complex compound was precipitated in the form of palladium ethylene diamine bichloride.
A mixture containing 0 g of nickel sulfate (NISO4-6H2O), so prepared from the reaction of nickel with sulfuric acid and 5 g of palladium ethylene diamine bichloride, so precipitated from the above process, was dissolved in 300 ml of distilled water and with the addition of amidosulfuric acid, the pH of the resulting solution was adjusted at 7.7 to prepare a plating solution. According to the chemical composition of palladium alloy, the weight percent ratio of palladium and nickel was 70:30.